Logging while drilling (LWD) techniques for determining numerous borehole and formation characteristics are well known in oil drilling and production applications. Such logging techniques include, for example, gamma ray, spectral density, neutron density, inductive and galvanic resistivity, micro-resistivity, acoustic velocity, acoustic caliper, physical caliper, downhole pressure measurements, and the like. Formations having recoverable hydrocarbons typically include certain well-known physical properties, for example, resistivity, porosity (density), and acoustic velocity values in a certain range. Such LWD measurements (also referred to herein as formation evaluation measurements) are commonly used, for example, in making steering decisions for subsequent drilling of the borehole.
LWD sensors (also referred to in the art as formation evaluation or FE sensors) are commonly used to measure physical properties of the formations through which a borehole traverses. Such sensors are typically, although not necessarily, deployed in a rotating section of the bottom hole assembly (BHA) whose rotational speed is essentially the same as the rotational speed of the drill string. LWD imaging and geo-steering applications commonly make use of focused LWD sensors and the rotation (turning) of the BHA during drilling of the borehole. For example, in a common geo-steering application, a section of a borehole may be routed through a thin oil bearing layer (sometimes referred to in the art as a payzone). Due to the dips and faults that may occur in the various layers that make up the strata, the drill bit may sporadically exit the oil-bearing layer and enter nonproductive zones during drilling. In attempting to steer the drill bit back into the oil-bearing layer (or to prevent the drill bit from exiting the oil-bearing layer), an operator typically needs to know in which direction to turn the drill bit (e.g., up or down). Such information may be obtained, for example, from azimuthally sensitive measurements of the formation properties.
In recent years there has been a keen interest in deploying LWD sensors as close as possible to the drill bit. Those of skill in the art will appreciate that reducing the distance between the sensors and the bit reduces the time between cutting and logging the formation. This is believed to lead to a reduction in formation contamination (e.g., due to drilling fluid invasion) and therefore to LWD measurements that are more likely to be representative of the pristine formation properties. In geosteering applications, it is further desirable to reduce the time (latency) between cutting and logging so that steering decisions may be made in a timely fashion.
One difficulty in deploying LWD sensors at or near the drill bit is that the lower BHA tends to be particularly crowded with essential drilling and steering tools, e.g., often including the drill bit, a near-bit stabilizer, and a steering tool all threadably connected to one another. LWD sensors commonly require complimentary electronics, e.g., for digitizing, pre-processing, saving, and transmitting the sensor measurements. These electronics are preferably deployed as close as possible to the corresponding sensors so as to minimize errors due to signal transmission noise and cross coupling. While the prior art does disclose the deployment of sensors in the drill bit (e.g., U.S. Pat. No. 6,850,068 to Chemali et al and U.S. Pat. No. 7,554,329 to Gorek et al) there is no suggestion as to how the above described problems can be overcome. Therefore, there is a need in the art for an improved drilling system that addresses these problems and includes a drill bit with at least one LWD sensor deployed therein.